It has long been known that iron ore in lump or pellet form can be efficiently and economically converted to sponge iron pellets in a vertical shaft, moving bed reactor by passing a hot reducing gas upwardly through a descending bed of the ore particles. It is also known that finely divided ore can be reduced to sponge iron in fine-grained form in a fluid-bed reduction reactor wherein the fine ore particles are suspended in a hot reducing gas. In general these two processes are essentially mutually exclusive since on the one hand the fine-grained ore, if used in a moving bed reactor, creates an excessive gas pressure drop through the bed and on the other hand, ores in lump or pellet form cannot be satisfactorily fluidized in a fluid bed reactor. The moving bed reactor has the important advantage that because it processes a high density mass of the ore it produces a substantially greater tonnage of product per unit volume of reactor than the fluid bed reactor.
It is usually desired to convert the sponge iron produced in such reduction processes to molten form and a number of methods of melting the sponge iron have been proposed. Thus U.S. Pat. Nos. 4,238,226 and 4,248,626 disclose moving bed reactors provided with melter-gasifiers in which a bath of molten iron is maintained. The product sponge iron pellets from the reduction reactor are fed to the top of the molten bath and the bath is maintained molten by feeding a mixture of pulverized coal and oxygen to the gasifier. Reducing gas generated in the gasifier can be used to reduce the ore in the moving bed reactor. Molten iron is intermittently removed from the gasifier for use in steel making. A generally similar system is shown in U.S. Pat. No. 4,007,034.
Processes such as those described above wherein sponge iron pellets from a moving bed reactor are fed to the top of a melter-gasifier are subject to a number of disadvantages due, in large measure, to the fact that the sponge iron contains components that form a layer of slag that floats on the surface of the molten iron bath. Because of its porosity sponge iron has a lower density than the slag and hence has a tendency to accumulate on the top of the slag layer rather than penetrate the slag layer and enter the underlying molten bath. This tendency can be at least partially overcome by providing a relatively long free-fall path for the sponge iron pellets to cause them to acquire sufficient kinetic energy to penetrate the slag layer. However, this necessitates an increase in the gas space above the molten bath and in any case does not insure that all of the sponge iron pellets will penetrate the slag layer.
It should further be noted that the failure of a substantial portion of the sponge iron to quickly penetrate the slag layer results in excessive cooling of the slag with a concomitant build-up of sponge iron both in and on the slag. Such a build-up can result in erratic furnace operation. Moreover, once the porous sponge iron pellets have entered the molten bath, their relatively poor heat conductivity and large particle diameter will retard the desired heat transfer and chemical reaction rates. This will increase the melting and gasification times and consequently the energy consumption due to thermal losses from the melter-gasifier.
It is known that during the reduction process in a vertical shaft, moving bed reactor a certain amount of sponge iron fines are produced by thermal and mechanical degradation of the iron ore. Hence another disadvantage of overhead continuous feeding systems as described above is that part of the fines may not reach the bath but rather may be swept out with the generated gas, thereby reducing the yield of molten iron and overloading the solids collection system through which the generated reducing gas passes after leaving the melter-gasifier. In addition, the presence of sponge iron fines in the upper part of the melter-gasifier can lead to severe chemical attack on and degradation of certain classes of refractory linings used in such equipment.
With respect to the liquefaction of fine sponge iron produced in fluid bed reduction processes, U.S. Pat. No. 4,045,214 describes a process wherein fine-grained iron ores are initially preheated and partially reduced in a counter-current heat exchanger, then reduced in a fluidized bed reactor to form sponge iron. The fine sponge iron is mixed with coal dust and the mixture fed to a molten iron bath in a melter-gasifier to which elemental oxygen is also fed to react with the coal dust and generate a reducing gas that is used as a fluidizing medium in the fluidized bed reactor. As pointed out above, such fluidized bed processes are subject to the disadvantage that because of the expanded character of the fluid bed they produce a relatively small amount of product sponge iron per unit volume of reactor. U.S. Pat. No. 4,008,074 also describes a process wherein fine-grained sponge iron is fed to the molten bath of a melter-gasifier.